The present invention relates to a process for hydrotreating hydrocarbons, using a new type of supported catalyst characterized by a particular structure conferring thereto an exceptionally increased retention power, as compared with that of a catalyst of the prior art. The retention power of a catalyst is defined as the maximum proportion of solid deposits, expressed in grams per 100 g of fresh catalyst, that this catalyst can tolerate in its pore volume, without decrease of its activity below, for example, 20% of its initial value. The definition of the activity of a catalyst depends on the type of the contemplated chemical reaction. The above mentioned solid deposits are those occurring when certain reactants of the charge contacted with the catalyst are converted in contact with the active sites of the catalyst to solid products which precipitate on the spot and whose accumulation in the course of time results in plugging of the pores. This accumulation leads to the poisoning of the initially present catalytic sites and hinders the free circulations of the reactants and of the products through the pores.
These deposits appear, for example, when hydrotreating oil fractions containing organometallic complexes: it is known in the art that, in the presence of hydrogen, hydrogen sulfide and a hydrotreatment catalyst, most complexes are destroyed and their constitutive metal precipitates as a solid sulfide which adheres to the internal wall of the pores. This is particularly the case of the nickel, vanadium, iron and copper complexes which are naturally present in crude oils in variable proportion, depending on the origin of the oil, and which, upon distillation, tend to concentrate in the fractions of high boiling point, particularly in the residues. It is also the case of coal liquefaction products which contain metals, particularly iron and titanium. The general term hydrodemetallation is used to designate these destruction reactions of the organometallic complexes in hydrocarbons.
The solid deposits in the catalyst pores can accumulate up to complete plugging of a portion of the pores controlling the access of the reactants to a fraction of the interconnected pore system, so that this fraction deactivates even though the pores of this fraction are only weakly obstructed or even remain unmodified. This phenomenon can thus promote an early and important deactivation of the catalyst. It is particularly noticeable in the case of hydrodemetallation reactions carried out in the presence of a supported heterogeneous catalyst. Heterogeneous means not soluble in the hydrocarbon feed charge. It is found, in this case, that the peripheric pores plug faster than the central pores and also that the pore openings plug quicker than the other parts of the pores. The plugging of the pores goes with a progressive reduction of their diameter, which increases the limitation of the molecules diffusion and results in a greater pressure drop, thus in a greater heterogeneity of the deposit from the periphery to the inside of the porous particles, up to complete obstruction of the pores opening on the outside, which occurs very quickly: the access to the nearly unmodified internal porosity of the particles is then closed to the reactants and the catalyst prematurely deactivates.
The above phenomenon is well known as "pore mouth plugging". The proof of its existence and the analysis of its origins have been published several times in the international scientific literature, for example in "catalyst deactivation through pore mouth plugging" presented at the 5th international symposium on chemical reaction engineering at Houston, Tex., U.S.A., in March 1978, or again in "Effects of feed metals on catalyst ageing in hydroprocessing residuum" in Industrial Engineering Chemistry Process Design and Development, volume 20, pages 262 to 273 published in 1981 by American Chemical Society, or more recently in "Effect of catalyst pore structure on hydrotreating of heavy oil" presented at the National Congress of the American Chemical Society in Las Vegas, U.S.A., on March 30, 1982.
It has been proposed to improve the efficiency and the life time of the catalysts subjected to poisoning by pore mouth plugging by making use of bimodal catalyst carriers. A bimodal catalyst carrier is, by definition, a porous solid whose porosity is distributed in two types of pores of very different average diameters, in the form of families of interconnected macropores and micropores. The micropores have a mean diameter averaging that of a conventional monomodal catalyst carrier, for example, of 3 to 10 nanometers, and they are effective by their large specific surface multiplying the chances of contact of the reactive molecules with the catalytic sites of the surface. The macropores have an average diameter between, for example, 100 and 1000 times the average diameter of the micropores, and their object is to improve the passage of the reactive molecules throughout the pore network. The diffusion velocity through the macropores is greater for the latter molecules, and thus the presence of macropores allows a better distribution of their concentration in the pore system. The solid deposits thus distribute themselves more regularly in the pore system and a greater amount thereof can thus accumulate in the catalytic pore volume before the plugging of the pores located at the periphery of the particles results in a complete deactivation. It is thus apparent that the presence of macropores in a catalyst carrier increases the life thereof when it is subjected to poisoning by plugging of the pore mouths.
Bimodal catalysts are disclosed, for example, in the French Pat. No. 1 592 580, No. 2 234 184 and No. 2 285 177 and in the U.S. Pat. No. 4,115,248, No. 4,225,421 and No. 4,257,922.